Many aircraft include one or more auxiliary power units (APUs) to supplement the main propulsion engines in providing electrical and/or pneumatic power. An APU may also be used to start the propulsion engines. An APU is, in most instances, a gas turbine engine that includes a compressor, a combustion system, and a turbine. During operation, the compressor draws in ambient air, compresses it, and supplies the compressed air to the combustion system. The combustion system receives fuel from a fuel source and the compressed air from the compressor, and supplies high energy combusted air to the power turbine, causing it to rotate. The power turbine includes a shaft that may be used to drive a generator for supplying electrical and/or pneumatic power.
However, typical conventional combustion systems present several challenges. First, typical combustion systems with rotary fuel slingers can present challenges with respect to recirculation and flame stability. A combustion system includes a combustor in which the air from the compressor is combusted. In order to establish a stable flame and corresponding high energy combusted air, the combustor utilizes a primary zone. The primary zone is a low-velocity, fuel-rich region in which hot combustion products are recirculated to encourage stable burning of the incoming fuel and air mixture. Conventional combustors may use swirlers to achieve the desired recirculation pattern; however, swirlers cannot be used in a combustor with a rotary fuel slinger that introduces the fuel from the fuel source. Accordingly, conventional combustors with rotary fuel slingers may have difficulty producing the desired level of recirculation and flame stability.
In addition, cooling of conventional combustors of combustion systems can be difficult. Cooling typically is provided for the liners of the combustor because of the high temperatures generated inside the combustor. The temperature generated inside the combustor may reach over 3500° F. while the metals used in combustor construction are limited to 1700-1800° F. Effusion cooling is a widely used technique for protecting the liner walls of the combustor from hot combustion gases. This cooling technique involves providing the combustor wall with a plurality of small holes. A supply of cooling air is passed through the holes from the cooler side of the combustor wall to the side of the combustor wall exposed to higher temperatures. The cooling air actively cools the liner by convection as it passes through the holes and by forming a protective layer of cool air on the hot side after the cooling air is discharged. However, effusion cooling may introduce excessive air that compromises the recirculation pattern and flame stability within the combustor.
Accordingly, it is desirable to provide combustion systems for gas turbine engines with improved flame stability. In addition, it is desirable to provide combustors for gas turbine engines with recirculation air flow patterns that promote flame stability in combustor systems with rotary fuel slingers and effusion cooling. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.